I'm trying to descifrate the purpose of the 'flag' field for cameras..
For an upcoming project that I have in mind, I would like to incorporate a camera that be persistant, I mean, a camera that cannot be bypassed by any event.

TrRosetta stone document says that is possible to do this, if the first bit of the flag is set. In my tests, I couldn't be able to achieve this effect.
I think it's only present in TR4 ( I use TR3 ).
Whatever value I place in 'flag' field, there is not any big effect.

If someone is interested in to descifrate this clue, I will be honorified!

I don't remember any fixed camera in TR1-3. I don't think it's possible, as you said... but maybe the function was "hidden" in the code and can actually be supported by older engines (like if you put triangles in tr1-2: they are rendered, even if without proper collisions).

To set the first bit, you have to enter 1 (0000 0000 0000 0001 in binary)

I assumed the less significant bit as first, so I entered "1" in the flag field.
But nothing happens.
The only persistant camera examples from older engines I know are these from cutscenes. Then this camera triggered in Offshore Rig where Lara has no weapons, but I think it is triggered by a script set in TombPc.dat.

Also, it comes to my mind that this 'flag' value should work together with another value from a trigger?

I always thought this flag value is for the sink object
As I had problems with underwater currents , I played a little bit with the flag value, but nothing happened. The only thing I noticed was, that the triggered sector had a different sink number(shown in fexinspect) each time the value got changed.

What comes to my mind is to check camera per camera settings in original compiled levels to see if there is a change in the flag field.
If after checking various levels and the flag is empty, I will come to Topixtor's conclusion as a 'reserved' field for older engines!

One thing related to cameras that i found interesting is that, if you use a "look at item" action in a trigger, Lara will look at that item even with weapons drawn. I used it in one of my new levels to look at a door opening no matter what.

I was using Sapper's fexInspect tool, and it seems that for every camera entry the field 'flag' is set to zero, for both, tr2 and tr3 levels.

However, for Sink entries, the flag field stores values.

So the last thing I need to check is to set value "1" for flag field for a camera compiling a TR4 level, and make sure that what the document says is just for tr4.

Danath: Yes, "look at item" thing works that way, I have something in mind to implement in order to make my concept work, as last alternative.

Edit:

I just did a test compiling a level of tr4. Exactly, when the flag is marked with the value 1, the focus of the camera is frozen no matter what events the player triggers, such as drawing the weapons or pressing the 'look' button. So, it only works for tomb raider 4. The clue is solved, hope this be of help to someone else!